This invention relates to the treatment of copper foil that is produced to be laminated to form printed circuits. In particular, it is a treatment that resists undercutting and produces a stable laminate bond under both normal and elevated temperatures.
In the production of printed electronic circuits, it is a common practice to bond metal foil to substrate material, generally a synthetic polymer, and to subject the composite structure to an acid etching treatment to form the desired circuit. Because the adhesion between conventional metal foil and such a substrate material is normally weak, considerable effort has been directed in the past to treating the foil so as to increase its bond strength with the substrate. As a result of such efforts, treatments have been developed which result in the enhancement of bond on one or both sides of the copper foil by electrodepositing a dentritic copper layer on its surface and gilding the dendritic layer so that when coated with a hardenable polymeric material the treated surface will, in effect, grip the polymer and form a tenacious bond.
While techniques such as the foregoing have succeeded in improving bond strength to some degree, problems have arisen in connection with the lamination of such treated foil to insulating substrates. More specifically, copper foil which has been provided with a "copper-type" treatment of the foregoing type tends, after etching to form the desired printed circuit, to leave traces of solid residue on the surface of the exposed insulating substrate. This residue is referred to in the trade as laminate staining or discoloration and is a highly undesirable effect. This laminate staining probably occurs because the matte (treated) side of the foil is subjected during the laminating process to contact with the softened resin. Chemical reactions apparently take place between the copper and the resin components, producing products which are not readily soluble in etching solutions used in printed circuit applications and which, accordingly, remain on the laminate surface, causing staining.
These problems are resolved by treating the copper foil so as to produce a matte surface formed of a plurality of copper electrodeposits having certain defined characteristics and coating the matte surface with a thin layer of zinc which, when heated during the laminating process, will form a brassy layer with the underlying copper. Such a layer provides the treated foil with good bond strength and renders the laminate made from it etchable in a single bath to produce the desired printed circuit with acceptable laminate color characteristics. It has been found that the desired characteristics will be achieved if the copper foil is subjected to a treatment which comprises the application to the foil of at least two separate electrodeposited copper layers, each succeeding electrodeposited layer having a different mechanical structure from a preceding electrodeposited layer to present a treated surface having physical properties different from those of the latter. In other words, this treatment involves a plurality of electrolytic copper treating operations carried out in a plurality of treating tanks, each one being carried out under separate electroplating conditions. The first treatment involves the electrodeposition on the copper foil of a nodular powdery copper layer which is coarse and rough and weakly adherent to the base copper foil, followed by a second treatment involving the application of an electrodeposited locking or gilding copper layer which is not nodular in structure but which conforms to the configuration of the first layer. The first treatment layer is supplied to increase the bond strength of the copper foil so that it can be more advantageously bonded to a substrate to form a laminate for use in electronic printed circuits. This first treatment step is capable of increasing the bond strength of one-ounce foil to range from 10 to 11 pounds per inch of width of laminate, depending upon the particular conditions utilized in this first treatment step. The amount of copper deposited in this first layer should be about 3-5 and preferably about 4 gms./m.sup.2 of foil.
The second treatment step, that is, the application of the "locking" or "gilding" copper layer, does not reduce the bond strength supplied by the initial copper layer treatment, and ordinarily will increase such bond strength to about 12-13 lbs./In. of width of laminate. It does, however, reduce or eliminate the disadvantageous powder transfer characteristics which the foil otherwise would have as a result of the first treatment stage. The layer deposited in this second treatment stage should have a thickness such that this layer causes substantially no decrease in bond strength. For best results, the amount of copper deposited in this second step to achieve this goal should be about 3-7 and preferably about 5 gms./m..sup.2 of foil.
There is one other threat to the quality of a printed circuit that results from processes used in its manufacture. This is undercutting, which is the removal of the material under some or all of the foil that is protected by the photoresist that is applied to define the printed circuit. Removal of copper under the photoresist weakens bonding of the copper to the board. In extreme cases of undercutting, portions of the printed circuit may even become detached from the board. It is therefore necessary that the substances used to produce the gilding layer and the barrier layer withstand the etchant sufficiently to produce an acceptable amount of undercutting. Details of the processes described above are given in U.S. Pat. No. 3,857,681, "Copper Foil Treatment And Products Produced Therefrom," which is incorporated here by reference as if set forth fully.
If a copper foil is produced which laminates with a good bond, etches cleanly in all common etchants, resists staining during the lamination process, and undergoes zero or a negligible degree of undercutting during etching, one further problem awaits. Either finished board that is not etched or an etched board that has not yet had parts inserted may be stored to await further processing. During this storage, the exposed copper is subject to staining from oxygen, sulphur and other compounds in the atmosphere. It is therefore desirable to find a coating for the surface that resists tarnishing without impairing other desired properties of the surface.